Field of the Invention
The present invention relates to a method for driving a switch in a switched-mode power supply, in particular in a free-running switched-mode power supply, and to a switched-mode power supply.
Switched-mode power supplies serve for providing an at least approximately constant, load-independent output voltage for a connected load. To that end, a primary coil of a transformer is connected in series with a switch to a supply voltage, and a secondary coil that is inductively coupled to the primary coil is connected, usually via a rectifier configuration, to the load for providing the output voltage. The primary coil takes up energy when the switch is closed and outputs the energy to the secondary side when the switch is subsequently opened. The switch is driven according to drive pulses generated by a drive circuit. The energy which is taken up by the primary coil per switch-on period of the switch and subsequently output to the secondary coil for supplying the load is larger, the longer the switch remains closed. The power consumed by the switched-mode power supply corresponds to the time average of this energy taken up per switch-on period of the switch.
The switch-on periods of the switch are prolonged with rising power consumption of the load, in order that the voltage present across the load can be kept at least approximately constant. In order to set the duration of the drive pulses, a control signal dependent on the output voltage is formed. If the output voltage changes on account of a load change, then the control signal also changes in order, in this way, to be able to adapt the duration of the drive pulses to the changed load conditions.
In order to define the switch-on instants of the switch, it is known to evaluate the voltage present across the primary coil or the switch and to close the switch again after opening when the primary coil is free of energy or demagnetized, that is to say when the primary coil has output its stored energy to the secondary side. In the case of small loads with a low power consumption, when the drive pulses are very short and the energy taken up by the primary coil per switch-on operation is correspondingly low, this results in a high switching frequency.
The switching losses of the power supply unit increase with a rising switching frequency of the switch. Therefore, Published, Non-Prosecuted German Patent Application DE 197 32 169 A1 proposes a switched-mode power supply in which, in the case of a small power output of the switched-mode power supply, the switch is not immediately switched on when the primary coil is free of energy or demagnetized. Rather, in the known switched-mode power supply, the switch-on instants are also set in a manner dependent on the control voltage, the time duration between two switch-on instants being longer, the smaller the power output by the switched-mode power supply. In order to set the switch-on instants, in the known apparatus, the control signal is used to form a pulse-width-modulated signal whose duration depends on the amplitude of the control signal and which concomitantly determines the time interval between two drive pulses.
It is accordingly an object of the invention to provide a method for driving a switch in a switched-mode power supply and a switched-mode power supply that overcome the above-mentioned disadvantages of the prior art methods and devices of this general type, in which a rise in the circuit losses in the case of a low power output is avoided.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for driving a switched-mode power supply having a power switch connected in series with a primary coil of a transformer. The switched-mode power supply generates a control signal dependent on an output voltage of the switched-mode power supply. The method includes the step of detecting reference instants. The reference instants are first reference instants at which a power switch voltage present across the power switch corresponds to a first reference voltage value or second reference instants at which a coil voltage present across the primary coil corresponds to a second reference voltage value. The power switch is closed after the number of the reference instants detected has reached a comparison numerical value. The comparison numerical value is increased or decreased in dependence on whether the control signal exceeds an upper limit value or falls below a lower limit value.
The method according to the invention for driving the power switch in the switched-mode power supply, which is connected in series with the primary coil of the transformer, provides, after the opening of the switch, for reference instants at which a voltage present across the switch corresponds to a first reference voltage value or at which a voltage present across the switch corresponds to a second reference voltage value to be detected and for the switch to be closed only when the number of detected reference instants corresponds to a comparison numerical value. The comparison numerical value is increased or reduced depending on whether a control signal dependent on an output voltage exceeds an upper limit value or falls below a lower limit value.
The method according to the invention utilizes so-called free transformer oscillations, which are established if, after the demagnetization of the primary coil, the switch is not immediately closed again in order to supply the primary coil with energy again. The free transformer oscillations result from a resonant circuit that, in the case of free-running switched-mode power supplies, is formed by the primary coil and a snubber capacitor connected in parallel with the switch, or in series with the primary coil. The transformer oscillations bring about a periodic alternating voltage across the switch or the primary coil. In the method according to the invention, xe2x80x9czero crossingsxe2x80x9d of the periodic voltage present across the primary coil or across the switch are detected. In the text below, xe2x80x9czero crossingxe2x80x9d designates an instant at which the amplitudes of the periodic alternating voltages across the switch, or across the primary coil, in each case intersect the amplitude value about which they periodically oscillate, or which corresponds to the average value of the respective periodic alternating voltage. The primary coil is at least approximately free of energy during the zero crossings. The interval between the zero crossings is constant and depends on the frequency of the resonant circuit.
The method according to the invention utilizes the uniformly spaced zero crossings as a temporal reference for switching on the switch again. The number of zero crossings that occur before the switch is switched on again is dependent on the comparison numerical value that is set in a manner dependent on the control signal. In this way, the switch-on frequency can be reduced as the output power decreases.
The comparison numerical value is increased or reduced depending on whether the control signal exceeds an upper limit value or falls below a lower limit value. In one embodiment, in which the control signal increases with a decreasing output voltage, the comparison numerical value is reduced if the control signal exceeds the upper limit value. The reduction of the comparison numerical value leads to an increase in the switching frequency or to a shortening of the time duration until the next switch-on of the switch and thus counteracts a further decrease in the output voltage. If the control signal drops below the lower limit value, then the comparison numerical value is increased in order to reduce the switching frequency and thus counteract a further rise in the output voltage.
The control signal is preferably evaluated before the counting up of the comparison numerical value during an evaluation time duration that is longer than the period duration of a periodically changing load, the comparison numerical value being counted up only when the control signal remains below the lower limit value during the entire evaluation time duration. Periodically changing loads lead to a periodically fluctuating output voltage and a periodically fluctuating control signal. The evaluation of the control signal during an evaluation time duration which is longer than the period duration of the load changes, and increasing the comparison numerical value only when the control signal remains below the lower limit value during the entire evaluation time duration, prevents the comparison numerical value from being counted up when the control signal just drops momentarily. As a result, it is possible to prevent the comparison numerical value from jumping back and forth between two successive numerical values and to avoid a continually changing switch-on frequency. It is important to prevent a continually changing switch-on frequency in order to avoid disturbances in a connected load, for example disturbances in the picture geometry of a television set, or humming in the transformer.
The minimum value of the comparison numerical value is one, which is equivalent to the switch being switched on again directly after the demagnetization of the primary coil.
In accordance with one embodiment of the invention, only those zero crossings are detected at which the voltage across the switch decreases or at which the voltage across the primary coil rises. In the case of such zero crossings, the storage capacitor undergoes a discharge process, that is to say the energy stored in the capacitor decreases. The switch-on of the switch with the storage capacitor being discharged reduces the switching losses since the storage capacitor is completely discharged anyway when the switch is closed.
A switched-mode power supply according to the invention has a switch which is connected in series with a primary coil of a transformer and turns on or turns off according to drive pulses, a drive circuit for generating the drive pulses in a manner dependent on an enable signal, and a controller configuration at which a control signal dependent on an output voltage is available. In order to generate the enable signal, a circuit configuration is provided which has a detection circuit, which detects reference instants at which a voltage present across the switch corresponds to a first reference voltage value or a voltage present across the primary coil corresponds to a second reference voltage value. A first counter is connected to the detection circuit, the counter has an output at which a first counter signal is available. The circuit configuration for generating the enable signal furthermore has a second counter having a first input, at which a signal is present which is dependent on a comparison of the control signal with a lower limit value, and a second input, at which a signal is present which is dependent on a comparison of the control signal with an upper limit value. The second counter also has an output at which a comparison counter signal is available. The first counter signal and the comparison counter signal are fed to a first comparator configuration, which provides the enable signal in a manner dependent on a comparison of the first counter signal with the comparison counter signal.
In accordance with one embodiment of the invention, a further comparator configuration is provided, which provides a first comparison signal, which is dependent on a comparison of the control signal with the lower limit value, and a second comparison signal, which is dependent on a comparison of the control signal with the upper limit value.
In accordance with an added feature of the invention, the drive circuit receives the control signal and the enable signal. The drive circuit outputs the drive pulses in a manner dependent on the enable signal and the control signal.
In accordance with an additional feature of the invention, the switch-on instants of the drive pulses are dependent on the enable signal and the drive pulses have a duration dependent on the control signal.
In accordance with a concomitant feature of the invention, the circuit configuration has a flip-flop connected between the further comparator configuration and the second counter. The flip-flop has a first input, a second input, and an output. The first input receives a signal dependent on the first comparison signal and the second input receives a clock signal. The first signal fed to the first input of the second counter is dependent on an output signal of the flip-flop.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for driving a switch in a switched-mode power supply and a switched-mode power supply, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.